A DFT mechanistic study on [4 + 3] cycloaddition reactions of oxyallyl cations and five-membered cyclopentadiene derivatives. Issue 36 (4th September 2020)

• Record Type: Journal Article
• Authors: Amoah, Anthony; Tia, Richard; Adei, Evans
• Is Part Of: Tetrahedron. Volume 76:Issue 36(2020)
• Publication Date: 2020-09-04
• Subjects: Oxyallyl cation -- Cycloaddition reaction -- Compact transition state -- Extended transition state -- Computational study; Chemistry, Organic -- Periodicals; 547.005
• DOI: 10.1016/j.tet.2020.131422
• Languages: English
• ISSNs: 0040-4020

Abstract

Abstract: The mechanism and stereo-selectivity of [4 + 3] cycloaddition reaction involving oxyallyl cations and five-membered cyclodienes (cyclopentadiene, furan and thiophene) for the construction of seven-membered cycloadducts has been studied with DFT at B3LYP/6-311G∗∗ level. The reaction can go through a concerted addition pathway via compact or extended transition states, or through a stepwise pathway. The concerted pathway is slightly favored kinetically (by 4 kcal/mol), via the compact transition state for furan and thiophene and the extended transition state for cylopentadiene, leading to a cis-endo product (W-configuration). Even though the stepwise addition pathway is not completely ruled out, it is not selective as the formation of the trans and cis-endo products through stepwise addition are very close in energy. Generally, the cycloadduct with the U configuration is more stable than the W and sickle configurations, and thus the kinetically-favored W conformer is expected to flip into the themodynamically-favored U conformer. Graphical abstract: Image 1 Highlights: The Concerted pathway is preferred for electron-donating substituted oxyallyl cations with cyclodiene derivatives studied. In terms of stereochemistry, the endo (compact Ts) transition state is preferred over exo (extended Ts). The reaction is stereo-selective towards the cis -endo cycloadduct in the concerted pathway. The cis -exo and trans cycloadduct could be favored in the stepwise pathway. Dichloromethane and water solvation has a considerable effect on the magnitude of the activation energy.